1. Field of the Invention
This invention relates to digital cellular communication systems, and more particularly, to digital cellular communication systems where a receiving station adapts to the symbol rate of the transmitting station without consuming overhead.
2. Related Art
In cellular telephone systems, several conversations or other communications can take place simultaneously on a single carrier frequency. Interference between calls is avoided by multiplexing and/or encoding the signals for the various conversations, but cross-talk, in which one conversation spills into another conversation, is still a problem in such systems.
Orthogonal variable spreading factor (OVSF) codes have been adopted in third generation W-CDMA (wideband code division multiple access) wireless systems as channelization codes. One of the advantages is that the use of OVSF codes can reduce cross-talk, or multiple access interference (MAI), effectively.
Another advantage of OVSF codes is that OVSF codes can provide users with variable spreading length access and thus enable multi-rate adaptation. In other words, symbols having different lengths can be used at different times, depending on the circumstances. Multi-rate adaptation increases the capability of multimedia communication, which often has variable bit rate demand. Multi-rate adaptation also increases the ability to cope with time- and location-bearing conditions of the mobile, wireless channel environment.
Keeping the data transmission rate (chip rate) constant, a base station can use OVSF codes of different code lengths for transmitting symbols of different symbol duration. The received symbol's energy changes because energy is the product of power and the symbol duration (the OVSF code length in this case). MAI noise is unchanged because codes from different branches in the code tree are orthogonal irrespective of length. Therefore, the signal to noise ratio (SNR) or signal to information ratio (SIR) can be controlled effectively by applying an OVSF code of a particular length for a symbol, without suffering from MAI due to energy leakage from other channels.
This variable length characteristic provides stations the capability of increasing transmission efficiency by dynamically adjusting SNR or SIR and the symbol rate according to the channel environment and the quality of service (QoS) requirement of the application. However, the rate adaptation scheme must provide the receiver with a scheme for detecting the rate change.
Previous studies discussed rate adaptation using either rate information (RI) messages, or blind rate detection. Using RI messages consumes wireless resources, incurring additional overhead for rate adaptation. Since resources of wireless communication are limited, it is desirable to reduce the overhead needed for rate detection at the receiver.
To improve performance and reduce this overhead, a blind rate detection scheme using a Viterbi-decoder has been suggested. The variable-rate data can be block-encoded with a cyclic redundancy checksum (CRC) and then convolutionally encoded before being transmitted. The receiver, with the knowledge of possible bit rates, uses a Viterbi-decoder to retrieve the convolutional-coded frame data and compare it with the CRC to keep the packet's integrity. The receiver must know the possible end bit positions {nend} of coded frame data, and the trellis path of the soft decision Viterbi-decoder should end up at the zero state at the correct end bit position. The blind rate detection scheme works with fixed frame systems, in which coded data is placed in frames with a fixed time interval. If the source data rate is lower than the maximum transmission rate, however, only a partial frame is filled with data packets and the rest of the frame is empty (or idle). As a result, the transmission efficiency is not optimal due to the waste of slots in frames.